Solid propellant rocket motors are known in the prior art in which the casing for the propellant is fabricated from strong filaments in a matrix of a curable polymer. A casing made in this way is known as a composite or filament-wound casing. Because of the nature of the composite material, water vapor from the atmosphere, over a long period of time, can permeate the casing. Permeation of the casing with water vapor can cause the propellant bond to the motor case and/or intermediate liner between the case and propellant to fail. Such failure results in an unusable rocket motor. If used, the rocket motor can fail catastrophically. The presence of moisture in the casing also reduces the strength of the resin.
A metal foil has been the most effective known barrier against such water vapor or moisture permeation. The application of a metal coating of sufficient thickness (about 0.003 inches or 0.008 centimeters, minimum) to a rocket motor composite casing is difficult, however, and presents a number of problems. First, for reasons of safety, ion-vapor deposition of a metal to the composite casing followed by a plating process cannot be effected after the rocket motor casing has been loaded with propellant. Nor can such metallic coating processes be performed on the motor casing before loading of the propellant. This is because of degradation of the casing that could result from the processing after the casing has been certified after pressure testing. The coating cannot be applied before certification with pressure testing because the pressure testing would result in expansion of the casing sufficiently to tear and debond the metallic coating, thus rendering it useless as a barrier against moisture permeation.
A preformed metallic coating adhesively applied to the surface of a rocket motor composite casing after loading could provide an effective barrier to water vapor permeation through the casing. Attempts to use such preformed coatings, however, have also been beset with problems, particularly with respect to the application of such preformed coatings to the ends of the casing which normally comprise a hemispherical dome of generally spherical shape.
For the cylindrical surfaces of the casing, or other surface areas of regular shape, a preformed metal coating can consist of a metal foil adhesively-backed tape such as aluminum tape. The cylindrical and regular surfaces can be covered with the tape spirally wrapped around the case and overlapped sufficiently to effectively prevent the passage of water vapor.
The hemispherical domes on the ends of the casing, however, cannot be covered with foil tape. This is because the foil tape cannot be applied without wrinkles. Wrinkles can cause cracks in the tape that allow moisture vapor passage thus rendering the foil tape useless as a barrier against water vapor permeation.
Another factor requiring consideration in the use of a metallic moisture barrier is low weight. Any weight not absolutely required to cause the rocket motor to operate properly diminishes the efficiency thereof.
It has been proposed in the prior art to fabricate free standing metal shells for adhesive bonding to the dome ends of a composite cased solid propellant rocket motor. Such preformed shells are of complex shape and must have sufficient thickness to prevent the passage of moisture. They must also be of the lightest practical weight which is structurally strong enough to allow handling and adhesive bonding to the rocket motor case. Attempts made in the prior art to fabricate such dome covers or shells have not been successful. One technique that has been tried is spin forming. Spin forming is widely used to form complex metallic shapes and would be satisfactory except that the thickness of metal required to use this technique is approximately 0.060 inches, minimum. This is about ten times the thickness, and consequently, the weight that is desired for metallic shells to cover the dome ends of a composite cased solid propellant rocket motor.
In addition, the previously mentioned pressure testing of the composite pressure vessels, which occurs at an air or hydro pressure equal to 1.25 or more times the maximum expected operating pressure of the pressure vessel, can itself produce formation of small voids or micro cracks, fractures and the like in the motor casing leading to or permitting entry of moisture or water vapor into or through the casing.
Thus, there is a need and a demand for an improved method that can overcome the aforementioned difficulties that have been encountered in the prior art for preserving composite material cased solid propellant rocket motors against the deleterious effects of moisture or water vapor in the atmosphere. A further need is for a method of improving the moisture or water vapor barrier of composite material cased solid propellant rocket motors and for eliminating or substantially eliminating the voids, cracks and/or fracture in the motor casing produced by pressure testing of the composite pressure vessels. An additional desired result would be to provide a means for increasing the electrostatic discharge protection of a composite pressure vessel and an even more desirable result would be to provide a means for increasing both the moisture or water vapor barrier protection and the electrostatic discharge protection of a composite material cased solid propellant rocket motor.